U.S. patent application Ser. No. 143,870, filed Jan. 14, 1988, describes a method and apparatus for projecting a seamless display produced from multiple video projectors all focused on a single screen. The image from each projector is projected so that it overlaps a portion of the image from another projector. In order to eliminate the bright bands or seams which result in the areas where two images overlap, the brightness of the overlapping portions of the images is ramped. This is done using commercially available special effects generators. While, in theory, the uniform, even ramping function of a typical special effects generator, when applied to the edges of each image, would result in a smooth transition from one image to another, in practice, the brightness of the projected images is not smooth nor consistent. The image from a video projector becomes darker toward the edges of the image as a natural function of the lens system used, and has a number of bright and dark portions caused by normal irregularities in the signal, intermediate signal processors, the projector, the projector's phosphors, screen reflectance, and many other factors. These inconsistencies will vary from one video component to another, and even among different components with identical constructions. In addition, different types of projectors respond differently to the same amount of brightness modification. As a result, the apparent image produced by smoothly ramping the brightness of overlapping images usually has several light and dark bands and spots. Accordingly, there is a need for a smoothing device which allows a user to precisely adjust the smoothing curve with which video brightness signals are ramped throughout the overlapping region and in neighboring areas as well. Such a smoothing device should be able to compensate for anomalies in individual projection systems and for differences between projection system sensitivity.